The present invention relates to an antifouling and anticorrosion device which applies a high voltage potential between a titanium anode and the conductive surface of the hull of a ship. The high voltage and the small current in the ship's submerged hull surface effectively prevents adherence of marine organisms to the hull while simultaneously preventing corrosion of the hull.
The shipping industry has long faced a serious problem caused by the adherence of marine organisms to ship hulls. Such fouling of a ship's hull increases the operating cost of a ship and decreases its efficiency. Marine organisms which become attached to the hull must periodically be removed, thereby usually taking the ship out of operation for extended periods of time for dry dock maintenance. Also, if fouling is not prevented, sea organisms will continue to attach to the hull and will cause ever increasing operating costs associated with additional fuel requirements and decreased speeds.
The prior art teaches several ways of removing marine organisms, including barnacle growth, from a ship. Barnacles can be mechanically scraped from the ship while in dry dock. Cleaning machines have been developed having rotating brushes which can remove barnacles and other marine organisms from the hull.
Another method of overcoming the fouling problem has been to use highly toxic paints on the hulls of ships. Such paints retard the build up of marine growth on the hull. A toxic element in the paint, such as a compound of copper or mercury which is soluble in seawater, is controllably dissolved into the water to provide protection over several years. For example U.S. Pat. No. 3,817,759 contemplates the use of an antifouling coating comprising a polymeric titanium ester of an aliphatic alcohol since titanium is known to have good corrosion resistance and low water solubility which prevents premature leaching and exhaustion of the coating.
Another antifouling method described in the prior art has been to coat the hull with a metallic paint whose ions are toxic to marine life, i.e., copper, mercury, silver, tin, arsenic, and cadmium, and then to periodically apply a voltage to the hull to anodically dissolve the toxic ions into the seawater thereby inhibiting marine life growth. This method is taught in U.S. Pat. No. 3,661,742 and in U.S. Pat. No. 3,497,434.
Antifouling systems which rely on dissolution of toxic paint into the seawater have limited utility since the coating applied to the hull is depleted and the hull must be periodically repainted. This problem is made even more severe in those systems which make the hull anodic to force dissolution since this increases the rate of dissolution. This poses a potentially serious problem since once the hull is exposed it to will be dissolved, resulting in pitting or puncturing of the hull.
Various other apparatuses have been proposed which rely upon application of a voltage to the hull of the ship or provision for flow of current through the hull of the ship to retard growth of marine organisms on the hull. Some systems have proposed the electro-chemical decomposition of seawater causing gases to be produced near the submerged surfaces of the hull. Proponents of such systems maintain that the gases prevent the adherence of marine organisms such as barnacles, algae, etc. Others suggest that high current can cause shock and retard the growth of marine organisms on the hull. None of these systems, however, have proven commercially successful for reasons of cost and poor antifouling results. Examples of these systems are disclosed in U.S. Pat. No. 4,196,064 and the Russian Patent No. 3388.
Solution to the problem of fouling requires a full understanding of the phenomenon involved. Fouling occurs especially on stationary structures and on ships in port, and there is relatively little fouling of a ship's hull while underway in the open sea. Although not understood in all respects, the phenomenon of fouling apparently is encouraged by bacteria and colloidal particles which in water solution possess an electric charge. For instance, amino acids are negatively charged and in combination as protein molecules are attracted to a ship's hull which is normally positive with respect to the protein molecules. These materials provide the elements of the marine organism food chain and form the initial film which appears on a ship's hull and attracts further sea creatures.
After formation of the initial phase of the food chain on the ship's hull, bacteria will form on the hull surface in one to three days, followed by an algae slime in three to seven days. Protozoans are observed within one to three weeks and finally barnacles attach to the hull in three to ten weeks. Interruption of the food chain will prevent adherence of marine organisms such as barnacles.
Another problem related to fouling of a ship's hull which the shipping industry has long attempted to solve is that of corrosion. Corrosion normally occurs to underwater portions of a ship's hull because the seawater acts as an electrolyte and current will consequently flow, as in a battery, between surface areas of differing electrical potential. The flow of current takes with it metal ions thereby gradually corroding anodic portions of the hull.
Various techniques have been developed to prevent corrosion. Sacrificial anodes of active metals such as zinc or magnesium have been fastened to the hull. Such anodes, through galvanic action, themselves corrode away instead of the hull. Other systems use cathodic protection by impressed current. Such systems utilize long life anodes which are attached to the hull to impress a current flow in the hull. The result is that the entire hull is made cathodic relative to the anode, thereby shielding it from corrosion. Such systems operate at very low voltage levels.
One cathodic protection system found in the prior art utilizes a titanium anode plated with platinum. The platinum acts as the electrical discharge surface for the anode into the electrolytic seawater. No current is discharged from any surface portions of the electrode comprising titanium. This particular system impresses high current densities on the anode on the order of 550 amps per square foot. Since there is a high current flow from the platinum or other non-soluble anode metal, there is a very low potential. There is essentially no current flow from the surface of the titanium. An example of such a system is disclosed in U.S. Pat. No. 3,313,721.
A titanium alloy has in another prior art system been used as a sacrificial anode. A pure titanium anode cannot be successfully used as a sacrificial anode because of the dielectric oxide layer which forms on its surface unless a quite high voltage is applied to it. In U.S. Pat. No. 3,033,775 a titanium alloy is used with such elements as cobalt, nickel, maganese, zinc, tin or the like to effect a lowering of the polarization potential of titanium thereby making it a good sacrificial anode. Indeed it has long been recognized that pure titanium does not perform satisfactorily as a soluble or sacrificial anode material because of the electrically resistant oxide film that forms on its surface.
A final problem faced by those desiring to develop a successful antifouling system is hydrogen embrittlement of the ship's hull. When electrolytic action takes place close to the surface of the ship's hull, such as in some of those systems described above, hydrolysis of the seawater may occur. Such hydrolysis releases hydrogen ions which cause embrittlement of the ship's hull. Consequently, it is important in any antifouling system which is installed that the system not be operated at such high currents to cause hydrolysis of the water thereby releasing hydrogen. This problem has prevented others in the art from developing a high voltage antifouling device which can successfully prevent the adherence of marine organisms without causing hydrogen embrittlement.